Why in the world would you buy downmarket from the 2600K and go to the 2500K? You can OC a 2600K to 4.9 GHz. Now, the problem is that some processors can take the OC, and some can't. But that is due to individual characteristics of the chip. If, due to defects on the 2600K it can onlyy be OC'd to 4.0 GHz, you have a defective chip (at least from the aspect of over clocking).
The other major hardware player in OC'ing is the motherboard. Some mobos OC well, and others don't. I have an ASRock P67 Extreme6 and it OC's like a champ, but I have run accross threads that bemoan the weak overclocking of ASRock mobos. The difference in mobos of the same model is dependent on the individual chips.
I guess that your system is limited by the mobo rather than the CPU. The 2600K should be able to OC at about 100 MHz over a 2500K.